With the development of information equipment such as communications equipment, there has recently been a demand for lead-acid batteries having a high voltage and a high capacity as the batteries used in such equipment. At the same time, there has also been a demand to reduce the maintenance work of lead-acid batteries and, particularly, prolong their life.
To meet such demands, many proposals have conventionally been made on valve-regulated lead-acid batteries. Maintenance work reduction is made by causing an electrolyte to be substantially contained in the micropores of a positive electrode plate, a negative electrode plate and a mat separator made of glass fiber, and causing the negative electrode plate to absorb oxygen gas generated from the positive electrode plate in the last stage of charging. This can suppress the electrolysis of water, thereby preventing the loss of the electrolyte.
Meanwhile, in valve-regulated lead-acid batteries, a free electrolyte does not exist in some cases, or the amount of the electrolyte is limited to a minimum in other cases. Thus, when a battery is used, the battery can be placed in free orientations; for example, it may also be placed sideways for use. With respect to large-sized batteries, in particular, a plurality of batteries that are placed sideways with their terminals at the front may be electrically connected in series, to be used as a battery set.
As for the prolongation of battery life, the pressure exerted on the electrode plate group is usually heightened to press the positive electrode active material by the separator, thereby suppressing the expansion of the positive electrode active material and preventing the separation of the positive electrode active material. However, with an increase in battery size, it has become difficult to exert an adequate pressure on the electrode plate group and maintain such a state even when the material of the battery container is changed or the battery container walls are thickened for strengthening the battery container. Further, in recent years, long-life batteries with a lifespan of 10 years or more have been required.
In lead-acid batteries, the corrosion of their positive electrode current collector due to oxidation progresses as the period of use becomes longer. Consequently, the cross-sectional area of the positive electrode current collector is decreased, thereby impairing the conductivity of the whole positive electrode plate. This results in degradation in voltage characteristics during high rate discharge of the battery. If such corrosion of the positive electrode current collector proceeds further, this will ultimately lead to breaking of the positive electrode current collector itself. As a result, the battery capacity decreases rapidly, and the battery reaches the end of its life.
In valve-regulated lead-acid batteries, the oxygen gas generated from the positive electrode plate is absorbed by the negative electrode plate, as described above, so that the scattering and discharging of the oxygen gas to the outside of the battery is suppressed. However, if the amount of gas the negative electrode plate can absorb is smaller than the amount of the oxygen gas generated at the positive electrode plate, the oxygen gas inside the battery is discharged to the outside of the battery, resulting in a decrease in the electrolyte. In contrast, if the negative electrode plate has a sufficient oxygen-gas absorbing ability, the battery voltage lowers during charge and the charge current increases during constant voltage charge. This promotes the above-described corrosion of the positive electrode current collector, thereby shortening the battery life.
As a method for preventing such corrosion of the positive electrode current collector, it is known to use a Pb—Ca—Sn alloy for the positive electrode current collector and increase the Sn content in this alloy to make the alloy crystals finer. For example, Japanese Laid-Open Patent Publication No. Hei 11-40186 discloses that the Sn content in the alloy constituting the positive electrode current collector is set to 1.05 to 1.50% by mass, in order to suppress the corrosion of the positive electrode current collector which is remarkable when the ratio of the positive electrode active material amount to the negative electrode active material amount is set to a relatively small range of 0.69 to 0.75.
However, such adjustments in the ratio of the positive electrode active material amount to the negative electrode active material amount and the specific surface area of the negative electrode active material can suppress a thermal runaway due to increased trickle current and battery life shortening, but these techniques only are not sufficient in cases a long life of 10 years or more is required.
Further, when a plurality of such batteries are connected in series to form a battery set having a voltage high enough to back-up a commercial power source, if there are large variations in the gas-absorbing abilities of the negative electrodes of the batteries, the charge voltages of the batteries vary. In this case, some batteries of the battery set become insufficiently charged or overcharged, which become factors causing a rapid shortening of the life of the whole battery set.
In order to solve the above-mentioned problems, it is therefore an object of the present invention to provide a valve-regulated lead-acid battery that is particularly suitable for use as a back-up power source and that has a stable long life.